The Mandate of Heaven is an ancient Chinese belief and philosophical idea that Heaven (天; Tian) granted emperors the right to rule based on their ability to govern well and fairly. According to this belief, heaven bestows its mandate to a just ruler, the Son of Heaven, and withdraws it from a despotic ruler, leading to the overthrow of that ruler. The Mandate of Heaven would then transfer to those who would rule best. The fact that a ruler was overthrown was taken by itself as an indication that the ruler had lost the Mandate of Heaven.
So, the right answer is A The Zhou Dynasty’s justification for its leadership.
